Rotomoulding polyethylene and method for producing said rotomoulding polyethylene

ABSTRACT

The present invention relates to a method for producing rotomoulding polyethylene by fluidised bed gas phase polymerisation of ethylene. The present invention further relates to the improved rotomoulding polyethylene obtainable by the invention process.

The present invention relates to a method for producing rotomouldingpolyethylene by fluidised bed gas phase polymerisation of ethylene. Thepresent invention further relates to the improved rotomouldingpolyethylene obtainable by the invention process.

Processes for the co-polymerisation of olefins in the gas phase are wellknown in the art. Such processes can be conducted for example byintroducing the gaseous monomer and comonomer into a stirred and/or gasfluidised bed comprising polyolefin and a catalyst for thepolymerisation.

In the gas fluidised bed polymerisation of olefins, the polymerisationis conducted in a fluidised bed reactor wherein a bed of polymerparticles is maintained in a fluidised state by means of an ascendinggas stream comprising the gaseous reaction monomer. The start-up of sucha polymerisation generally employs a bed of polymer particles similar tothe polymer, which it is desired to manufacture. During the course ofpolymerisation, fresh polymer is generated by the catalyticpolymerisation of the monomer, and polymer product is withdrawn tomaintain the bed at more or less constant volume. An industriallyfavoured process employs a fluidisation grid to distribute thefluidising gas to the bed, and to act as a support for the bed when thesupply of gas is cut off. The polymer produced is generally withdrawnfrom the reactor via a discharge conduit arranged in the lower portionof the reactor, near the fluidisation grid. The fluidised bed consistsin a bed of growing polymer particles. This bed is maintained in afluidised condition by the continuous upward flow from the base of thereactor of a fluidising gas.

The polymerisation of olefins is an exothermic reaction and it istherefore necessary to provide means to cool the bed to remove the heatof polymerisation. In the absence of such cooling the bed would increasein temperature and, for example, the catalyst becomes inactive or thebed commences to fuse. In the fluidised bed polymerisation of olefins,the preferred method for removing the heat of polymerisation is bysupplying to the polymerisation reactor a gas, the fluidising gas, whichis at a temperature lower than the desired polymerisation temperature,passing the gas through the fluidised bed to conduct away the heat ofpolymerisation, removing the gas from the reactor and cooling it bypassage through an external heat exchanger, and recycling it to the bed.The temperature of the recycle gas can be adjusted in the heat exchangerto maintain the fluidised bed at the desired polymerisation temperature.In this method of polymerising alpha olefins, the recycle gas generallycomprises the monomer and comonomer olefins, optionally together with,for example, an inert diluent gas such as nitrogen or a gaseous chaintransfer agent such as hydrogen. Thus, the recycle gas serves to supplythe monomer to the bed, to fluidise the bed, and to maintain the bed atthe desired temperature. Monomers consumed by the polymerisationreaction are normally replaced by adding make up gas or liquid to thepolymerisation zone or reaction loop.

A gas fluidised bed polymerisation reactor is typically controlled toachieve a desired melt index and density for the polymer at an optimumproduction. Conditions within the polymerisation reactor have to becarefully controlled to reduce the risk of agglomerate and/or sheetformation which may ultimately lead to bed instabilities and a need toterminate the reaction and shut down the reactor. This is the reason whycommercial scale reactors are designed to operate well within provenstable operating zones and why the reactors are used in a carefullycircumscribed fashion.

It has now been found a new process for producing rotomouldingpolyethylene in fluidised bed gas phase reactor, wherein an improvedprocess-operating envelope is used. Thus, the present invention relatesto a process for producing rotomoulding polyethylene, having a density Acomprised between 930 and 944 kg/m3 and a melt index B comprised between3 and 7.8, by (co-)polymerisation of ethylene in a fluidised bed gasphase reactor, said process comprising

-   -   determining the instantaneous density d and melt index MI of the        polyethylene powder exiting the reactor,    -   allowing the density and melt index to vary around their A and B        values by a value of plus or minus 3 kg/m3 for the density and        plus or minus 30% for the melt index,    -   characterised in that the operating temperature is controlled        such that    -   1. the RTSE factor is first maintained in the        operating-enveloppe corresponding to the d and MI values of the        polyethylene produced, and    -   2. the RTSE factor is maintained between 4.2 and 4.4.

The present invention further relates to a rotomoulding polyethylenegrade having a density comprised between 930 and 944 kg/m3 and a meltindex comprised between 3 and 7.8 characterised in an ESCR propertyequal or higher than 400 h and a Charpy property equal or higher than 10kJ/m2. Preferably, the said polyethylene is a non-metallocene containingpolymer.

The ESCR can be measured according to ASTM-D-1693.

The Charpy can be measured according to ISO 179-2. Preferably, theCharpy value is equal or higher than 14 kJ/m2.

The densities can be measured according to ASTM-D-792 and defined as inASTM-D-1248-84. The rotomoulding polyethylene of the present inventionhas a density comprised between 930 and 944, preferably between 933 and941 kg/m3.

The melt index can be measured according to ASTM-D-1238, condition A(2.16 kg). The rotomoulding polyethylene of the present invention has amelt index comprised between 3 and 7.8, preferably between 3 and 7 g/10min.

According to the present invention, the density and melt index arerespectively allowed to vary around their A and B values by a value ofplus or minus 3 kg/m3 for the density and plus or minus 30% for the meltindex. It means, for example, that for a rotomoulding polyethylene withA=937 and B=5, acceptable variations are for density from 934 to 941 andfor melt index from 3.5 to 6.5. According to a preferred embodiment ofthe present invention, the density and melt index are respectivelyallowed to vary around their A and B values by a value of plus or minus2 kg/m3 for the density and plus or minus 15% for the melt index.

The RTSE factor is indicated in the attached tables (FIG. 1 to 20). ARTSE value comprised between 4.2 and 4.4 corresponds to eachdensity/melt index couple. To every RTSE corresponds an operatingtemperature. For density or melt index values that are falling at theborder of operating enveloppes (window), the corresponding operatingtemperature enveloppe can easily be calculated by making linearinterpolations. For example, in FIG. 1, for a 932/3.7 density/melt indexcouple, the operating temperature at an RTSE of 4.3 is the averagebetween 96.2° C. (i.e. operating temperature for a 932/3.8 density/meltindex couple at RTSE of 4.3) and 96.6° C. (i.e. operating temperaturefor a 932/3.6 density/melt index couple at RTSE of 4.3), i.e. 96.4° C.As already indicated, the invention is characterised in that theoperating temperature is controlled such that the RTSE factor is firstmaintained in the operating-enveloppe corresponding to the D and MIvalues of the polyethylene produced, and the RTSE factor is maintainedbetween 4.2 and 4.4.

According to a preferred embodiment of the present invention, during theproduction of a specific rotomoulding polyethylene grade, the RTSEfactor is allowed to vary only by plus or minus 0.07, preferably 0.05across the operating enveloppes, said variation occurring within aminimum of 4 hours of operation, preferably within a minimum of 8 hoursof operation.

This process is preferably applied during the fluidised bed gas phasepolymerisation of olefins, and may also advantagesouly be used duringstart-up and especially during product grade transition between tworotomoulding polyethyene.

The instantaneous density and melt index properties correspond to theproperties of the resin formed instantaneously in the reactingconditions at a given time. The “instantaneous properties” are differentfrom the pellet properties which correspond to a mixture of differentresins formed continuously in the fluidised bed (averaging effect).

The process according to the present invention is particularly suitablefor the manufacture of copolymers of ethylene. Preferred alpha-olefinsused in combination with ethylene in the process of the presentinvention are those having from 4 to 8 carbon atoms. The preferredalpha-olefins are but-1-ene, pent-1-ene, hex-1-ene, 4-methylpent-1-ene,oct-1-ene and butadiene, the most preferred comonomer being thehex-1-ene.

When liquid condenses out of the recycle gaseous stream, it can be acondensable monomer, e.g. but-1-ene, hex-1-ene, 4-methylpent-1-ene oroctene used as a comonomer, and/or an optional inert condensable liquid,e.g. inert hydrocarbon(s), such as C₄-C₈ alkane(s) or cycloalkane(s),particularly butane, pentane or hexane.

The process is particularly suitable for polymerising olefins at anabsolute pressure of between 0.5 and 6 MPa and at a temperature ofbetween 85 and 115° C., preferably between 90° C. and 110° C.

The polymerisation is preferably carried out continuously in a verticalfluidised bed reactor according to techniques known in themselves and inequipment such as that described in European patent application EP-0 855411, French Patent No. 2,207,145 or French Patent No. 2,335,526. Theprocess of the invention is particularly well suited to industrial-scalereactors of very large size.

The polymerisation reaction may be carried out in the presence of acatalyst system of the Ziegler-Natta type, consisting of a solidcatalyst essentially comprising a compound of a transition metal and ofa cocatalyst comprising an organic compound of a metal (i.e. anorganometallic compound, for example an alkylaluminium compound).High-activity catalyst systems have already been known for a number ofyears and are capable of producing large quantities of polymer in arelatively short time, and thus make it possible to avoid a step ofremoving catalyst residues from the polymer. These high-activitycatalyst systems generally comprise a solid catalyst consistingessentially of atoms of transition metal, of magnesium and of halogen.The process is also suitable for use with Ziegler catalysts supported onsilica. The process is also especially suitable for use with metallocenecatalysts in view of the particular affinity and reactivity experiencedwith comonomers and hydrogen. The process can also be advantageouslyapplied with a late transition metal catalyst, i.e. a metal from GroupsVIIIb or Ib (Groups 8-11) of the Periodic Table. In particular themetals Fe, Co, Ni, Ru, Rh, Pd, Os, Ir, and Pt are preferred, especiallyFe, Co and Ni. The late transition metal complex may comprise bidentateor tridentate ligands, preferably coordinated to the metal throughnitrogen atoms. As examples are those complexes disclosed in WO96/23010.Suitable iron and/or cobalt complexes catalysts can also be found inWO98/27124 or in WO99/12981.

It is also possible to use a high-activity catalyst consistingessentially of a chromium oxide activated by a heat treatment andassociated with a granular support based on a refractory oxide.

The catalyst may suitably be employed in the form of a prepolymer powderprepared beforehand during a prepolymerisation stage with the aid of acatalyst as described above. The prepolymerisation may be carried out byany suitable process, for example, polymerisation in a liquidhydrocarbon diluent or in the gas phase using a batch process, asemi-continuous process or a continuous process.

According to a preferred embodiment of the present invention, thecatalyst is a Ziegler-Natta catalyst (i.e., non-metallocene) containingmagnesium and titanium; the magnesium is preferably acting as thesupport; the catalyst is thus preferably non supported on silica.Preferably, the catalyst is subjected to a prepolymerisation stage. Amost preferred catalyst corresponds to the catalysts disclosed inWO9324542.

According to a preferred embodiment of the present invention, thepolyethylene has a density comprised between 930 and 944 kg/m3 and amelt index comprised between 3 and 7.8. The polyethylene is preferablyan hex-1-ene copolymer of ethylene. It has preferably an ESCR propertyequal or higher than 400 h, more preferably higher than 500 h. It haspreferably a Charpy property equal or higher than 10 kJ/m2, morepreferably equal or higher than 15 kJ/m2. The molecular weightdistribution is preferably comprised between 3 and 8, more preferably3.5 and 5.

According to a more preferred embodiment of the present invention, thepolyethylene has a density comprised between 930 and 944 kg/m3 and amelt index comprised between 5 and 7.8. The polyethylene is preferablyan hex-1-ene copolymer of ethylene. It has preferably an ESCR propertyequal or higher than 750 h. It has preferably a Charpy property equal orhigher than 15 kJ/m2. The molecular weight distribution is preferablycomprised between 3 and 8, more preferably 3.5 and 5.

The said polyethylene is preferably a non-metallocene containingpolymer.

EXAMPLES

The polymerisations are carried out continuously in a vertical fluidisedbed reactor as described in the example of European patent applicationEP-0 855 411.

The catalyst (prepolymerised Ziegler-Natta catalyst) used in all thefollowing examples is prepared according to the procedure disclosed inexample 1 of WO9324542.

Comparative examples 1 to 3 correspond to non RTSE polymerisationconditions.

Examples 4 to 9 correspond to RTSE polymerisations conditions.

The data are given in the below table. Notched Charpy Molecular ImpactDensity (kg/m3) Weight Resistance Polymerisation MI (2.16 kg) ASTM D792& Distribution ESCR (h) (B) (kJ/m2) Ex. Comonomer Temperature (° C.)ASTM D1238 ASTMD1248-84 (MWD Mw/Mn) ASTM D1693 ISO 179-2 C1 Butene-1 833.1 943.2 4 185 9 C2 Butene-1 83 4.1 938.0 4 200 9 C3 Butene-1 83 6.0935.2 4.5 900 13 4 Butene-1 108 3.0 943.3 4 230 9 5 Butene-1 101 3.9938.1 4 290 10 6 Butene-1 96 6.2 934.8 4.5 1150 15 7 Hexene-1 108 3.3942.9 4 400 14 8 Hexene-1 101 4.2 937.8 4 550 15 9 Hexene-1 96 6.3 935.14.5 1550 22

1. Process for producing polyethylene for rotomoulding, having a densityA comprised between 930 and 944 kg/m3 and a melt index B comprisedbetween 3 and 7.8, by (co-)polymerisation of ethylene in a fluidised bedgas phase reactor, said process comprising determining the instantaneousdensity d and melt index MI of the polyethylene powder exiting thereactor, allowing the density and melt index to vary around their A andB values by a value of plus or minus 3 kg/m3 for the density and plus orminus 30% for the melt index, characterised in that the operatingtemperature is controlled such that
 1. the RTSE factor is firstmaintained in the operating-enveloppe corresponding to the d and MIvalues of the polyethylene produced, and
 2. the RTSE factor ismaintained between 4.2 and 4.4.
 2. Process according to claim 1 whereinthe density and melt index are respectively allowed to vary around theirA and B values by a value of plus or minus 2 kg/m3 for the density andplus or minus 15% for the melt index.
 3. Process according to any of thepreceding claims wherein the RTSE factor is allowed to vary only by plusor minus 0.07 across the operating envelope(s), said variation occurringwithin a minimum of 4 hours of operation.
 4. Process according to claim3 wherein the RTSE variation is only allowed within a minimum of 8 hoursof operation.
 5. Process according to claims 3 and 4 wherein the RTSEfactor is allowed to vary only by plus or minus 0.05 across theoperating envelope(s).
 6. Process according to any of the precedingclaims wherein it applied during product grade transition between twopolyethyene for rotomoulding.
 7. Process according to any of thepreceding claims wherein the polyethylene is a copolymer of ethylene andhex-1-ene.
 8. Process according to any of the preceding claims whereinthe polyethylene for rotomoulding has a density comprised between 933aid 941 kg/m3 and a melt index comprised between 3 and 7 g/10 min. 9.Polyethylene grade for rotomoulding obtainable by any of the precedingclaims and having a density comprised between 930 and 944 kg/m3 and amelt index comprised between 3 and 7.8 characterised in an ESCR propertyequal or higher than 400 h and a Charpy property equal or higher than 10kJ/m2.
 10. Hex-1-ene copolymer of ethylene having a density comprisedbetween 930 and 944 kg/m3, a melt index comprised between 3 and 7.8, anESCR property equal or higher than 400 h and a Charpy property equal orhigher than 10 kJ/m2.